I'm going to assume you meant to put CoS (for Circle of Scorn) instead of RoS since there isn't a PLD/WAR ability with that acronym.

Quote Originally Posted by LeoLeoLeo View Post
On the first mageadd I build up with the first two moves of the RoH combo on Levi's tale, and then on mage - RoH - RoS - FB - SW - SB - stun - FB - stun - SB - stun

On the second I start with provoke - shield lob - RoS - FB - sw - SB - stun - FB - stun
I wouldn't go with a rotation mainly because the trigger for his dreadstorm cast (which is the dangerous one that can't be stunterrupted) is based upon his hp as well as time (much like Levi's phases). It'll start casting dreadstorm either after darkness (~10 seconds after spawn) or as soon as it hits 80% (it will finish the action it's currently performing but will cast as soon as that's over).

Which trigger you have to react to depends entirely upon your group's DPS (in this case, initial burst DPS) so you should try and figure out how much damage it's likely to take within the next GCD. You can pretty much always get off Lob>FB>SB; with good DPS, you won't be able to get in the RoH. If you're not sure if you should attack, don't. Tank DPS isn't substantial, and stunning the add is way more important than getting in an extra 300-400 damage.

The second interrupt is best used right as your first stun is running out since the add is generally still in its "use dreadstorm asap" phase. The third should be held in reserve to interrupt dreadwash since it only lasts 1 second so it's only good for the interrupt. If the add isn't dead by the time you get that third stun it, you've basically got bad DPS (often, this means that you had one or more DPS ignoring the add).

The only way that a PLD can fail at stunlocking is if the add gets off dreadstorm or dreadwash before your third stun. If the add gets off a cast otherwise, it was your DPS that failed.